Radiotelephone transmitters generally include an automatic gain control loop to maintain a desired transmission power level out of the antenna. Without such a gain control loop, the transmission power might vary as a function of factors such as transmission frequency, battery voltage, and/or temperature.
In particular, a portion of the power generated by the transmitter power amplifier can be coupled through a capacitor to a rectifier where it is converted to a voltage. This voltage can then be digitized and compared to a calibrated digital code at the radiotelephone microprocessor. A control signal from the microprocessor is used to adjust a variable gain amplifier when the digitized voltage is not equal to the calibrated digital code to maintain the desired transmission power.
Because a portion of the power generated by the transmitter is coupled through the feedback loop, however, the efficiency of the transmitter is reduced. Because the power amplifier generates more power than is actually transmitted through the antenna, the battery life may be reduced. A lower coupling capacitance may thus provide lower coupling and higher efficiency. Proper operation of the feedback loop, however, may dictate a certain level of coupling and the resulting inefficiency.
Moreover, a radiotelephone transmitter may operate at one of a plurality of transmission power levels depending on a signal strength desired at the time. For example, radiotelephones according to the IS-136 standard (IS-136 dual-mode dual band radiotelephones) may transmit at one of eight different transmission power levels ranging from 28 dBm to -4 dBm. Choosing a capacitance value for the coupling capacitor, however, may be difficult over this range of transmission power levels. In particular, a lower value coupling capacitance provides lower coupling and thus higher efficiency, but the lower capacitance may couple insufficient power at the lower power levels to maintain operation of the feedback loop. Higher capacitor values may provide sufficient power coupling at lower power levels for operation of the feedback loop, but the reduced efficiency may significantly reduce the radiotelephone battery life.
U.S. Pat. No. 5,363,071 to Schwent et al. discusses a method for varying coupling of a radio frequency signal. This is accomplished by selecting between a first operating mode of the RF coupler having strong coupling, responsive to a first predetermined power level, such that the RF coupler produces a first coupled RF signal responsive to the RF signal and a second operating mode of the RF coupler having weak coupling, responsive to a second predetermined power level, such that the RF coupler produces a second coupled RF signal responsive to the RF signal. The strongly and weakly coupled RF signals, however, may still contribute to undesired transmission inefficiencies.
Furthermore, it may be difficult to provide a cost effective analog-to-digital converter having useful resolution over the range of power levels discussed above. For example, a 32 dB range of output powers may be difficult to measure using a conventional 8 bit analog-to-digital converter. In conventional arrangements, the highest code of the analog-to-digital converter is set at the maximum operating voltage of the converter, and the lowest code is set at ground. Accordingly, there is a voltage range near the supply voltage and ground where the digital code is variable or redundant. For example, if the supply voltage is 3.3V and there is a 300 mV range of uncertainty, the analog-to-digital converter may only be able to reliably convert from 0.3V to 3V providing a dynamic range of: EQU .vertline.x.vertline.=20 log(0.3/3)=20 dB.
For the reasons discussed above, it may be difficult to provide useful feedback at lower power levels. Accordingly, radiotelephones may effectively provide open loop transmit operations at the lower power levels thereby reducing performance thereof. Furthermore, known feedback loops may result in undesired transmission inefficiencies.